The present application relates to laser-based spectroscopy, and more particularly, but not exclusively, relates to noise reduction and absorbance estimation for spectroscopy based on frequency modulation of a laser.
Various techniques for detecting extremely low concentrations of a substance of interest have been developed that involve Frequency Modulation (FM) of a laser beam. FM techniques include those described as Frequency Modulation Spectroscopy (FMS), wideband FM, Wavelength Modulation Spectroscopy (WMS), and the like; by Joel Silver in Frequency-Modulation Spectroscopy for Trace Species Detection: Theory and Comparison Among Experimental Methods, APPLIED OPTICS, vol. 31, No. 6, pp.707–717 (20 Feb. 1992), which is hereby incorporated by reference.
Regardless of the particular type, for these techniques the frequency-modulated laser beam is typically directed through an analyte that is characterized by spectral absorption and/or dispersion of the interrogating light. The returned light, an altered form of the interrogating light, is detected and evaluated to determine a certain spectroscopic characteristic of interest. Frequency modulation of laser light typically results in a coincidental amount of Amplitude Modulation (AM), so-called Residual Amplitude Modulation (RAM), due to nonideal behavior of the laser and/or other elements of the system. RAM is often regarded as undesirable because of limits it can impose on the sensitivity of FM techniques.
To enhance sensitivity of laser-based spectroscopic detection, a double beam approach has been implemented that uses a reference light beam to cancel noise present in the interrogating light beam. For this approach, carefully matched detectors and light beam paths are often needed, which can lead to considerable expense and complexity. Moreover, various remote sensing applications cannot practically incorporate multiple, long-range beams. In some remote sensing applications that may be particularly desired for homeland security, pollution control, and the like; a single beam system can suffer from noise induced by atmospheric turbulence, optical component vibration, and/or one or more other noise sources that likewise alter intensity of the laser beam. Such “intensity noise” can significantly effect sensitivity.
Accordingly, there is a need for further contributions in this area of technology.